Systems and methods are known for manufacturing polymeric products, such as sheets, films or tubes, wherein a liquid polymer or resin is forced under pressure through an extrusion die. The extrusion die generally comprises an annular orifice for blowing the liquid resin into a tubular film. The tubular film is inflated and blown into a cylindrical bubble which eventually congeals downstream of the blowing head. The location at which the blown film bubble congeals is referred to as the frost line, or crystallization point. The blown film bubble, in the congealed state, is subsequently collapsed and flattened farther downstream and is formed into a flat sheet. This flat sheet may be rolled and used in the manufacture of mass quantities of plastic bags or a variety of other sheet like products.
In order to ensure that the flat film product meets quality control standards for properties such as film width and uniformity of the film thickness, various parameters associated with the film blowing process may be controlled. For example, the rotational speed of the extruder screw may be controlled in order to adjust the speed of ejection of the resin from the die head such that the frost line is located and maintained at or near a particular distance above the die head. Also, the die gap settings and the flow of external cooling air which is blown around the outer periphery of the film bubble may be controlled such that the film is blown at a desired thickness which is uniform around the entire circumference of the film bubble. Thus, the extruder apparatus may be controlled in order to ensure that these properties are maintained at or near predetermined ideal values.
Systems are known for controlling the film blowing process in order to maintain predetermined ideal conditions of the blown film bubble. Such systems are disclosed in Upmeier et al, U.S. Pat. No. 4,246,212; Halter, U.S. Pat. No. 4,189,288; and Havens, U.S. Pat. No. 4,101,614. These systems provide measurements of the film properties in a target area on the film bubble and control the film blowing process in order to maintain the film properties in that target area at or near the predetermined values. Havens discloses a temperature sensing device which is focused to read and continually monitor the film temperature at a target point on the bubble, wherein the target point is located below the frost line and above the extruder die head. Upmeier also discloses a sensing device for measuring the temperature of the bubble in a control area which is in the vicinity of the actual frost line position. The temperature at this point is compared to the known crystallization temperature for the resin being used and the system is controlled to maintain the measured value at the control set point value.
However, several problems associated with these systems have been recognized. Although, monitoring the temperature of the bubble and controlling the process parameters in order to maintain the measured temperature at a desired set point value is a desirable method of maintaining the quality of the film product, measuring the temperature at a single point target area provides inadequate results. Temperature measurement at a single point on the bubble is not representative of the mean temperature around the circumference of the bubble, and control based on such a single point measurement does not take into account overall mean changes in the process. Accordingly, control actions made in response to such a measurement may not be effective to produce an end product of a desired quality. Also, Havens recognizes that a target area in the vicinity of the frost line is not a good control area, as crystallization effects occurring at or near the frost line cause the temperature measurements in that area to be unstable and uncharacteristic of the changes below the frost line. Finally, a comparison between the temperature measured in a target area in the vicinity of the frost line and the predicted crystallization temperature for the particular resin being used may be unsatisfactory, as differences in resin composition and blend may cause the actual crystallization temperature to vary from the predicted value.
These references also disclose means for measuring the film thickness and uniformity and controlling the extruder output parameters in order to maintain the film thickness and uniformity at or near the predetermined desired values. However, as these systems are responsive to temperature input from single point temperature sensing devices, the film thickness and uniformity may not be adequately determined based upon this film temperature measurement at only a single point on the bubble. Thus, additional apparatus for the measurement of these properties of the film are necessary, adding to the complexity and cost of the system.
Therefore, there is a need for an improved system for controlling the extruder apparatus process parameters in accordance with measurements of the film temperature simultaneously taken at multiple points vertically up and down and around the circumference of the bubble. The present invention provides an improved system and method which satisfies this need.